This invention relates to magnetic resonance imaging which makes use of nuclear magnetic resonance (NMR). More particularly, the invention relates to methods and apparatus for magnetic resonance vascular imaging adapted for obtaining a high-contrast image of a part including a flow such as the cardiovascular system.
Stated briefly, magnetic resonance imaging methods comprise the steps of positioning an object to be investigated in a magnetic field in order to align unpaired nuclear spins parallel to the field and irradiating the object with radio frequency (RF) excitation pulses in order to thereby tip the spins by an angle (or the flip angle).
Magnetic resonance angiography techniques such as the time-of-flight (TOF) methods and the phase-sensitive methods have been known for producing images with vascular contrast by making use of the flow of blood in vessels. The time-of-flight methods are so called because they make use of the time-of-flight effects of the motion of blood flow, as will be explained below. The phase-sensitive methods make use of motion-induced phase shift effects.
According to a time-of-flight method, a slab-like region (hereinafter referred to as an excitation slab) 4 of thickness L perpendicular to a vessel 5 is selectively excited first as shown in FIG. 5. After time TR, the same slab-like region is again selectively excited by a pulse. Since blood is flowing inside the vessel 5 at a speed of V, a part of the blood which was earlier excited has flowed out of the excitation slab 4 as shown in FIG. 6 by the time the next excitation pulse is applied. Since a new supply of fully relaxed blood (not yet excited) enters this excitation slab 4 in the meantime, a maximum signal can be obtained from such a not-yet-excited blood portion if the flip angle of the excitation pulse is 90.degree.. In a situation, as shown in FIG. 6, where a portion of the earlier excited part of the blood is still remaining on the downstream side inside the excitation slab 4, however, it is an excitation pulse of a small flip angle that generates a maximum signal from this portion. This situation of having earlier excited blood remaining occurs most prominently where the downstream side connects to a peripheral vessel. In such a situation, the time-of-flight effect will be different between the upstream and downstream sides within the same excitation slab 4. FIG. 8 shows the prior art profile of flip angle, that is, the prior art distribution of the flip angle in the direction of the thickness of the excitation slab 4 indicated as the Z-direction in FIGS. 5 and 6. A high-contrast image of the entire vessel 5 cannot be obtained if the flip angle is uniform along the direction of the flow of blood as shown in FIG. 8.
By the phase-sensitive methods, too, excitation must be repeated many times for imaging. If an excitation pulse of same flip angle is applied to both the freshly introduced portion of the blood which has not been excited yet and the remaining portion of the blood which has been excited many times already, there arises the problem of not being able to optimize the signal intensity from the blood from all parts if the slab.